The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile system which forms a color image with the use of electrophotography, and in particular, to an electrophotographic apparatus for forming a color image with the use of a plurality of color toners.
With the electrophotography, a light beam is projected from an exposure means onto a photoconductive medium which is uniformly charged, so as to form a latent image corresponding to image data, and toner is stuck to the latent image on the photoconductive medium so as to develop the latent image. The thus obtained toner image is transferred onto and fixed on a recording medium.
It is noted here that explanation will be hereinbelow made so as to use the recording medium as a sheet. However, the recording medium should not be limited to the sheet alone but there may be used a sheet-like recording medium made of any of various materials including plastic.
In order to form a color image, a plurality of color toners such as yellow Y, magenta M, cyan C and black K are superposed one upon another so as to form the image.
There are two kinds of color image forming systems, such as a repeated developing system in which development is repeated on a single photoconductive medium with color toners so as to form a color image, and a simultaneous developing system in which developments are simultaneously carried out on a plurality of photoconductive mediums with color toners so as to form a color image.
The repeated development system is the one in which a single photoconductive medium is used for forming a color image, and as a typical example, there an intermediate transfer medium system.
In the intermediate transfer medium system, a plurality of developing means for developing images with different color toners are arranged around a photoconductive medium, together with an intermediate transfer medium, and toner color images formed on the photoconductive medium are transferred one by one onto the intermediate transfer medium (Refer to, for example, JP-A-8-137179). This transfer is repeated for different color images so as to superpose these images on the intermediate transfer medium in order to form a color image which is then transferred onto a medium on which the color image is fixed.
In the intermediate transfer medium system, since different toner color images of, for example, yellow Y, magenta M, cyan C and black K are formed one by one on the photoconductive medium, and are then transferred onto the intermediate transfer medium, being superposed one upon each other, a time which is four times as long as that required for formation of a monochromatic image is required.
The simultaneous developing system simultaneously forms color toner images respectively on a plurality of photoconductive mediums for respective different colors, and transfers the different color toner images in association with a conveyance of a sheet so as to form a color image. Thus, this developing system is also called as a tandem system. (Refer to, for example, JP-A-2001-356548).
The tandem system incorporates an image forming means including a photoconductive medium, a charging means, an exposure means, a developing means and a cleaner means, independently, for each color, and accordingly, four image forming means are required for forming a color image with color toners of yellow Y, magenta M, cyan C and black K.
In the tandem system, different toner color images are formed by four independent image forming means, simultaneously in parallel with one another, and are then transferred onto an intermediate transfer medium or a sheet. In the tandem system, since different toner color images are simultaneously superposed one upon another, a color image can be formed by a time nearly equal to that required for formation of a monochromatic image, and accordingly, this system is preferable for high speed printing of a color image.
These years, there have been increased demands for colorization of documents in offices and accordingly, color printers have been rapidly spread in use. Further, it has been desired to increase the printing speed, and accordingly, tandem system color printers have been spot-lighted.
However, since the tandem system color printer inevitably incorporates four image forming means, the miniaturization of the printer is difficult, that is, it has a size which is relatively larger than that of a repeated development system color printer.
In order to make the apparatus small-sized, there may be utilized either a manner in which the height of the apparatus is decreased so as to flatten the apparatus or a manner in which the floor area of the apparatus is decreased so as to have a vertical type having an increased height. In the case of installation of a printer in an office or a home, the restraint to the height thereof is relatively less, and accordingly, configuration having an increased height but a decreased floor area is desirable if the bulk of the printer is fixed.
Further, these years, in order to save paper resource, a double face printing function for printing opposite surfaces of a paper sheet has been desired, and accordingly, it is required to make the apparatus small-sized while incorporating a double face printing function.
In case of the color printer of the repeated development system, four color toner images of yellow, magenta, cyan and black which are to be printed on the backside of a paper sheet, are formed one by one on a photoconductive medium, and are then superposed on an intermediate transfer medium, and accordingly, there is a time for reversing the sheet for the preparation of transferring the images onto the backside of the sheet. Thus, no difference is appreciated in printing speed between single face printing and double face printing.
On the contrary, in the case of the color printer of the tandem system, when images are printed successively on one side of a paper sheet, they are successively printed on the paper sheet one by one with a space of about, for example, 50 mm therebetween.
However, in the case of double face printing, a sheet on which single face printing has been completed is reversed while a conveying direction is reversed so that the tailing end of the sheet is turned into the leading end, and the sheet whose advancing direction is reversed is inserted in a conveying path upstream of a transfer means in order to transfer a full color image which is formed on the intermediate transfer medium, onto the backside of the sheet. Thereafter, the full color image is fixed. Thus, since the toner image cannot be transferred onto the sheet when the sheet is reversed, the printing speed per minute of the double face printing is lower than that of the single face printing.
Accordingly, it has been desired to increase the printing speed of the double face printing so as to approach the printing speed of the single face printing as possible as it can.
There has been known an electrophotographic apparatus which can form an image while a sheet is conveyed in a substantially vertical direction during double face printing (Refer to, for example, JP-A-2001-002330).
In this photographic apparatus, there are provided, in an openable door, a sheet conveying means for downward conveying sheet in a substantially vertical direction while transferring toner images onto the sheet along a plurality of image forming means, a sheet reversing and conveying means as a reversing means for reversing the sheet to be subjected to double face printing so that the leading end of the sheet is turned into the trailing end thereof, a guide portion for reversing the advancing direction of the sheet fed upward from the reversing and conveying path by an angle of about 180 deg., so as to downward direct the same, a sheet refeeding means for conveying the sheet from the curved guide portion to a position which is off from a pair of registering rollers to a sheet cassette.
In this electrophotographic apparatus, the fusing means is arranged above the image forming means, that is, it is located in the uppermost part of the apparatus.
Meanwhile, the registering rollers at a stating point of the image forming portion for transferring images onto a sheet, are located in the lowermost part of the conveying path, near the sheet cassette.
In order to print the backside surface of a sheet for which the transfer and fusing of the image has been completed on the front side thereof, the sheet is inserted downward into the reversing and conveying path so that the traveling direction of the sheet is reversed, and accordingly, the end of the sheet which has been the trailing end until then is turned into the leading end thereof. Thus, the images are transferred onto the sheet after the sheet is merged into the conveying path upstream of the registering rollers.
Further, there has been known an electrophotographic apparatus incorporating a main conveying path and a bypass conveying path, a recording medium is reversed through the bypass conveying path, and is returned into the main conveying path through which the recording medium is again conveyed with its backside surface facing to the imaging forming portion (Refer to, for example, JP-A-6-208266).
In the electrographic apparatus disclosed in JP-A-2001-2330, the return conveying path for returning a sheet fed from the reversing conveying path for registering rollers is provided semicircular guide portions at its top and bottom since the sheet fed out upward has to be turned by an angle of about 180 deg. so as to be conveyed downward to a position in the vicinity of the lower end of the apparatus, and then has to be turned by an angle of about 180 deg, so as to be conveyed upward before it is merged into the conveying path upstream of the registering rollers.
Should the radius of each of these curved part portions be decreased to a small value, it would be likely to cause jamming of a sheet, and accordingly, it cannot be sufficiently decreased, that is its possible minimum diameter should be about 50 mm.
Thus, the thickness of the opening door has to be about 70 mm at minimum since there is required a space for mounting conveying rollers to the reversing conveying path and the sheet refeeding means, in addition to the diameters of the curved guide portions, thereby there has been a limitation to miniaturization of the electrophotographic apparatus.
Further, in this conventional technology, the opening door portion is provided therein with a sheet conveying means, a sheet refeeding means and, a seat reversing means which are stacked in three layers, and accordingly, the thickness of the opening door becomes larger.
In the electrophotographic apparatus as disclosed in JP-A-6-208266, in the case of printing the backside surface of a sheet, since no extra space corresponding to a reversing conveying path which temporarily stores therein a sheet, is required, the configuration thereof is appropriate for miniaturization.
However, in order to carry out double face printing, since a sheet is conveyed through the main conveying path in a reverse direction, a printing sheet as a next page cannot be soon fed until the reversal is completed, there is a limitation to increasing of a printing speed of double face printing.